gcc/doc/portability.texi - gcc - Git at Google

 @c Copyright (C) 1988-2021 Free Software Foundation, Inc.
 @c This is part of the GCC manual.
 @c For copying conditions, see the file gcc.texi.

 @node Portability
 @chapter GCC and Portability
 @cindex portability
 @cindex GCC and portability

 GCC itself aims to be portable to any machine where @code{int} is at least
 a 32-bit type.  It aims to target machines with a flat (non-segmented) byte
 addressed data address space (the code address space can be separate).
 Target ABIs may have 8, 16, 32 or 64-bit @code{int} type.  @code{char}
 can be wider than 8 bits.

 GCC gets most of the information about the target machine from a machine
 description which gives an algebraic formula for each of the machine's
 instructions.  This is a very clean way to describe the target.  But when
 the compiler needs information that is difficult to express in this
 fashion, ad-hoc parameters have been defined for machine descriptions.
 The purpose of portability is to reduce the total work needed on the
 compiler; it was not of interest for its own sake.

 @cindex endianness
 @cindex autoincrement addressing, availability
 @findex abort
 GCC does not contain machine dependent code, but it does contain code
 that depends on machine parameters such as endianness (whether the most
 significant byte has the highest or lowest address of the bytes in a word)
 and the availability of autoincrement addressing.  In the RTL-generation
 pass, it is often necessary to have multiple strategies for generating code
 for a particular kind of syntax tree, strategies that are usable for different
 combinations of parameters.  Often, not all possible cases have been
 addressed, but only the common ones or only the ones that have been
 encountered.  As a result, a new target may require additional
 strategies.  You will know
 if this happens because the compiler will call @code{abort}.  Fortunately,
 the new strategies can be added in a machine-independent fashion, and will
 affect only the target machines that need them.
	@c Copyright (C) 1988-2021 Free Software Foundation, Inc.
	@c This is part of the GCC manual.
	@c For copying conditions, see the file gcc.texi.

	@node Portability
	@chapter GCC and Portability
	@cindex portability
	@cindex GCC and portability

	GCC itself aims to be portable to any machine where @code{int} is at least
	a 32-bit type. It aims to target machines with a flat (non-segmented) byte
	addressed data address space (the code address space can be separate).
	Target ABIs may have 8, 16, 32 or 64-bit @code{int} type. @code{char}
	can be wider than 8 bits.

	GCC gets most of the information about the target machine from a machine
	description which gives an algebraic formula for each of the machine's
	instructions. This is a very clean way to describe the target. But when
	the compiler needs information that is difficult to express in this
	fashion, ad-hoc parameters have been defined for machine descriptions.
	The purpose of portability is to reduce the total work needed on the
	compiler; it was not of interest for its own sake.

	@cindex endianness
	@cindex autoincrement addressing, availability
	@findex abort
	GCC does not contain machine dependent code, but it does contain code
	that depends on machine parameters such as endianness (whether the most
	significant byte has the highest or lowest address of the bytes in a word)
	and the availability of autoincrement addressing. In the RTL-generation
	pass, it is often necessary to have multiple strategies for generating code
	for a particular kind of syntax tree, strategies that are usable for different
	combinations of parameters. Often, not all possible cases have been
	addressed, but only the common ones or only the ones that have been
	encountered. As a result, a new target may require additional
	strategies. You will know
	if this happens because the compiler will call @code{abort}. Fortunately,
	the new strategies can be added in a machine-independent fashion, and will
	affect only the target machines that need them.